There remains a continuing need for polyester resin compositions which have excellent color and clarity and also have improved reheat rates in the reheat blow-molding process. There are a variety of reasons why polyester resins, and polyethylene terephthalate (PET) in particular, suffer from poor color and clarity. First, the infrared absorbing compounds typically used to increase the resin's reheat rate, such as carbon black, antimony metal, black iron oxide, red iron oxide, inert iron compounds, spinel pigments, and infrared absorbing dyes, tend to have a negative effect on the visual appearance of PET containers by increasing the haze level and/or causing the article to have a dark appearance (i.e. a decreased L* value). Second, the antimony compounds that are typically used as polycondensation catalysts for PET tend to form insoluble antimony complexes which degrade the resin's color and clarity or are reduced to metallic form which impart a gray color. Third, catalysts that are used to prepare PET can also catalyze degradation reactions within the polymer, thus producing trace impurities which increase the yellowness of the polymer (i.e. a higher b* value).
The amount of absorbing compound that can be added to a polymer is limited by its impact on the visual properties of the polymer, such as brightness, which may be expressed as an L* value, and color, which is measured and expressed as an a* value, a b* value, and haze, as further described below.
To retain an acceptable level of brightness and color in the preform and resulting blown articles, the quantity of reheat additive may be decreased, which in turn decreases reheat rates. Thus, the type and amount of reheat additive added to a polyester resin may be adjusted to strike the desired balance between increasing the reheat rate and retaining acceptable brightness and color levels.
Due to aesthetic reasons, a blue tint is sometimes desired in polyester beverage containers, especially containers for water applications. Polymer articles with a blue tint tend to be more appealing to the human eye, and are thus generally preferred in these applications. Yellowness, which may be measured as a b* value in the CIE color system, may thus be a particularly undesirable color in consumer packaging, and bluing agents such as cobalt and organic toners have been used to increase the blue tint of consumer packaging, thus shifting the b* value from yellow to blue (or from higher to lower b* values), creating a more appealing package. It would be ideal to simultaneously increase the reheat rate and decrease the rate at which color and brightness degrade, such as by increased yellowness, as the concentration of the reheat additive in a thermoplastic composition is increased.
Highly active catalysts which improve the reaction rate are problematic because they also accelerate degradation reactions which impart yellowness. Titanium-based polycondensation catalysts are typically known to be highly active and to impart a yellow hue to the polymer. To control the yellow hue, polyester polymers are made using small amounts of titanium catalysts at low reaction temperatures. Even with such precautions, the yellow hue of the polymer is increased to some extent. Polyester resins catalyzed with lithium and aluminum catalysts are also highly active but normally impart a yellow hue to the polymer.
It would be desirable to provide a polyester resin having a low yellow hue or a more neutral b* color, low haze, with good reheat rates and made with highly active catalysts.